‘Nuts n’ tails’ fluid testing is promising, practical method of PRRSV monitoring

Download our special report

This is one of eight reports that appears in a special edition
of Pig Health Today, “Framing the Future of PRRS.”
For a free copy, click here.

Testing fluids obtained from routine castration and tail-docking procedures looks to be a promising, practical and affordable way to improve monitoring for porcine reproductive and respiratory syndrome virus (PRRSV), reported Daniel Linhares, DVM, PhD, from Iowa State University (ISU).

The fluids used for testing are aggregate samples — each sample has fluids from several pigs — comprised of serosanguineous drainage gathered during castration and tail docking, Linhares said.

The procedure, which Linhares said was developed by his PhD student Will Lopez, DVM, is simple:

• A bucket is lined with a disposable plastic bag, followed by a piece of cheesecloth.
• A large rubber band is placed around the outside top of the bucket to secure the bag and cheesecloth.
• Removed testicles and tails are placed in the bucket so their fluids drain through the cheesecloth into the plastic bag.
• The plastic bag is removed and a small hole is made in the bottom of the bag so the fluids can be funneled into testing vials.

“Put them on ice and you’re ready to submit the samples to the lab,” Linhares said. The lab treats the samples similarly as it would treat serum and tests for PRRSV RNA by quantitative polymerase chain reaction (qPCR). The ISU lab is currently working on optimizing the diagnostic assays used to detect viral RNA and antibodies from processing-fluid samples, he noted.

MONTHLY SERUM-TESTING PROBLEMS

The veterinarian cited problems with the current method of statistical sampling that depends on conventional individual pigserum testing. Serum taken monthly from 30 individual piglets provides a 95% probability of detecting PRRSV when the prevalence is 10% or higher. However, it is well established that the chance of detecting PRRSV based on individual blood samples is poor when the prevalence of the virus is low, Linhares said.

He said he’s often asked if the likelihood of detecting PRRSV at near-zero prevalence could be improved by blood sampling 30 or 60 pigs weekly or bi-weekly instead of monthly. “The short answer is no,” Linhares emphasized, because the same population isn’t being sampled.

“Pigs are born every day. You wean every week. Sows move in and move out. So, those results don’t add up. To improve the probability of finding virus when the prevalence is low, we need to sample more pigs, more frequently.”

Conventional sampling of more pigs more frequently, however, isn’t practical considering that trained personnel are required and that it’s a two-person, time-consuming and costly job to get blood from individual pigs.

RESULTS WITH FLUIDS, SERUM COMPARED

The herd sensitivity of processing-fluid testing is still being evaluated, but so far, it appears to be not only simpler but better than serum testing, he said.

To compare results, Linhares, Lopez and colleagues intentionally obtained weekly samples from several breeding farms where they expected to have a PRRSV prevalence of about 10%.

For each sampling, they collected blood serum from 30 pigs and divided it into six pooled samples, each with blood from five pigs. They also obtained one aggregate fluid sample from all pigs castrated and tail docked on each sampling day. In total, they had 10 sets of processing-fluid samples from the farms to compare to 10 sets of serum samples, Linhares said, noting that up to 600 piglets contributed to each of the processing-fluid samples.

Only one qPCR test was required to test each aggregated, processing-fluid sample compared to six qPCR tests needed to test serum samples from 30 pigs. “Overall, only 20% of the blood-serum samples tested positive compared to 100% of the processing-fluid samples (Table 1),” Linhares said.

In addition, anti-PRRSV antibodies using an IDEXX ELISA were detected in processing fluids, he said.

Linhares and colleagues hope that testing processing fluids can help detect PRRSV even when the prevalence is low. Toward this end, they plan to evaluate this monitoring method on farms with a low PRRSV prevalence.

WINDOW OF OPPORTUNITY

Testing of processing fluids, Linhares continued, could help producers resolve a common problem on farms undergoing PRRSV elimination: Piglets are born negative for the virus but test positive at weaning.

The animals might be infected due to management practices, pig or people movement and other factors between birth and weaning. That leaves a 10- to 12-week period of time when producers have a window of opportunity to prevent horizontal transmission of PRRSV to piglets.

What he called “nuts n’ tails” monitoring could make it easier to pinpoint when pigs are becoming infected and when farms need to get aggressive about correcting problems so that pigs born PRRSV-negative remain so at weaning.

Linhares said he and colleagues are also conducting a retrospective study aimed at identifying the best control strategies to reduce the production impact of PRRSV. Their preliminary results indicate the impact is greater in systems that permit cross-fostering after 24 hours, in systems with poorly cleaned farrow-barn hallways and alleys and due to some gilt-acclimation practices.

The impact of the virus on production is less in herds with previous PRRSV exposure, with PRRSV-monitoring programs in place and when there is a PRRSV immunization strategy, he said.